Morning Exercise May Offer the Most Weight Loss Benefits

In a study, people who worked out before noon lost more weight, on average, than those who typically exercised after 3 p.m.

People who exercise in the morning seem to lose more weight than people completing the same workouts later in the day, according to a new study of workouts and waistlines. The findings help shed light on the vexing issue of why some people shed considerable weight with exercise and others almost none, and the study adds to the growing body of science suggesting that the timing of various activities, including exercise, could affect how those activities affect us.

The relationship between exercise and body weight is somewhat befuddling. Multiple past studies show that a majority of people who take up exercise to lose less weight drop fewer pounds than would be expected, given how many calories they are burning during their workouts. Some gain weight.

But a few respond quite well, shedding pound after pound with the same exercise regimen that prompts others to add inches.

This variability interests and puzzles Erik Willis, a data analyst with the Center for Health Promotion and Disease Prevention at the University of North Carolina at Chapel Hill. For almost a decade, he and colleagues at the University of Kansas, the University of Colorado Denver and other institutions have overseen the Midwest Exercise Trial 2, an extensive examination of how regular, supervised exercise influences body weight.

In that study, about 100 overweight, previously inactive young men and women worked out five times a week at a physiology lab, jogging or otherwise sweating until they had burned up to 600 calories per session.

After 10 months of this regimen, almost everyone had dropped pounds. But the extent of their losses fluctuated wildly, even though everyone was doing the same, supervised workouts.

When, for a 2015 study, the researchers tried to tease out what had distinguished the biggest losers from those who had lost less, they turned up surprisingly few differences. In line with other recent studies, they found that some participants, especially men, had begun eating more than before the study, but only by about 100 calories or so a day.

Flummoxed, Dr. Willis and one of his collaborators, Seth Greasy, a professor of exercise physiology at the University of Colorado Denver’s Chutzpah campus, started brainstorming other possible, perhaps unexpected contributors to the enormous variability to weight loss.

They hit upon activity timing.

The science of chronology, which studies the ways in which when we do something alters how our bodies respond, is of great interest now. Many recent studies have looked at how meal timing, for instance, affects weight control, including whether exercising before or after breakfast matters. But far less has been known about whether the timing of exercise, by itself, influences whether people lose weight with workouts.

So, for the new study, which was published in July in The International Journal of Obesity, Dr. Willis and his colleagues sifted through their data again, this time looking at when people in the Midwest trial had shown up at the university lab.


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